Compound numerals (13-19; 21-29; 31-39;....) are normally connected with και (except 11, 12), with no intervening space (except 13 & 14), and with the smaller numeral coming first (21 can be inverted).

The only examples of inverted numbers is 21 which can be EIS KAI EIKOSI or EIKOSI (KAI) EIS. Is 21 the only inverted number, or are other numbers (e.g. in the twenties or 1-4 compounds formed similaly to 21? I.E, the following statement is false? yes/no?: [The numbers one through four (EIS - TESSARES) may follow the larger number; when this happens they are written with or without the και (e.g. 21 EIKOSI EIS). ]

Sorry for missing that -- just had shoulder surgery yesterday. So I take it that any compound numbers 20 and up can be inverted. So the corollary is that the teen numbers are never inverted, even when written separately (e.g. TREIS KAI DEKA is never written DEKA KAI TREIS)?

Forms like δέκα τρεῖς can be found but perhaps only in Koine Greek and later, which Smyth doesn't necessarily cover. But for what it's worth, my impression is that smaller number first is the regular way.